WM,
I doubt that your cables have broken or stretched so that you cannot open your cover with the key.
The problem is caused by a small plastic hook inside the cable pull assemby that is attached to the bottom of the key lock assembly. When this happens even the front emergency pull will not unlatch the front cover. If the problem is on the rear cover half, there is no emergency pull, but the cause is still the same. In fact, in over 5 years on this web site, I have not heard of anyone who actually broke a cable. The cable pull hook is in the bottom of the plastic housing where the cables attach, and the hook is about as thick as your fingernail. It is very easy to break of the very tip of the hook and it no longer will engage the cable pull drum that pulls the cables. So the key will turn, but the hook cannot rotate the barrel/drum that pulls the cables.
You can remove the by using a pair of channel lock pliers to twist and pull the plastic cable pull housing from the bottom of the key lock cylinder. You will need to replace the cable assembly. You can also cut the rivets off the plastic cable pull housing and get the cables out of the housing. Then you can pull the cables with pliers until you get your new cable assembly.
...Rich